The present invention relates to an optical integrated device, a semiconductor laser module and an optical transmitter, particularly to a technology effectively applied to a technology for stabilizing output of laser beam radiated from a plurality of semiconductor lasers.
In WDM (Wavelength Division Multiplexing) optical communication, it is very important to adjust and maintain wavelengths of light sources having a plurality of different wavelengths respectively to predetermined wavelengths prescribed by Communication Standards. Therefore, there have been proposed various wavelength adjusting technologies. For example, there is disclosed a technology of controlling a laser oscillation wavelength of a semiconductor laser (laser diode: LD) in Japanese Patent Laid-open (Kokai) No. Hei 9-129959.
There is adopted normally a method of transmitting laser beams emitted from respective semiconductor lasers to a separately packaged wavelength monitoring package by using optical fibers and feeding back control signals in correspondence with deviations from predetermined wavelengths to the respective semiconductor lasers.
According to the method, the apparatus becomes large-sized. In recent years, there has been proposed a method of mounting a small-sized optical output and wavelength monitor using an etalon filter at a vicinity of a position of emitting light of a semiconductor laser chip and including the monitor in a single package to thereby constitute a semiconductor laser module. For example, there is disclosed a wavelength monitor (optical monitor integrated optical transmitter module) for wavelength division multiplexing optical fiber communication in Japanese Patent Laid-open (Kokai) No. Hei 10-79723. According to the apparatus, there is constructed a constitution of arranging DFB (distributed feedback)-LD in a pin package, irradiating laser beam to outside of the package by passing forward beam of the laser diode through a lens, an isolator and a lens and guiding backward beam to a PIN array via Fabri-Perot etalon.
The package, for example, the package in a box shape, is described in A. Kasukawa, xe2x80x9cHigh Power Semiconductor Laser for Exciting a Fiber Amplifierxe2x80x9d, Kogaku (Japanese Journal of Optics), Vol. 29, No. 3, 2000, pp. 148-151, Optical Society of Japan (in Japanese). According to the literature, there is described a laser module (pig-tail module) projecting a plurality of leads (external electrode terminals) to align from both sides of a box-type package in a shape of a parallelepiped.
Further, according to Japanese Patent Laid-open (Kokai) No. Hei 7-50443, there is described a distributed feedback type laser with a modulator. According to the literature, there is described a selective growth technique as a method of integrating optical semiconductor elements having different kinds of functions such as a semiconductor laser, an optical modulator, an optical switch, an optical detector and an optical amplifier on the same semiconductor substrate.
According to the conventional optical monitor integrated optical transmitter module, there is constructed a structure integrated to a single package, which is an excellent method for making apparatus small-sized and inexpensive. However, according to the method, a single laser diode is formed on a semiconductor chip (semiconductor laser chip).
In the meantime, in recent years, there has been proposed a chip (semiconductor laser chip) in which in order to emit a plurality of wavelengths of beams in a range at least covering a portion of a plurality of wavelengths prescribed by the wavelength division multiplexing optical fiber communication, not only a laser diode having a predetermined single wavelength but also a plurality of semiconductor lasers (laser diodes) are monolithically integrated on a single semiconductor substrate, further, a multiplexer for multiplexing forward beams of the lasers, an amplifier and a modulator are monolithically integrated (K.Kudo et al., 25th Optical Fiber Communication Conference, TuL5, (2000)).
Hence, the inventors have investigated to combine the above-described integrated semiconductor laser chip with a small-sized optical monitor and integrating thereof to a box-type package, conventionally used. However, according to the method, it is known that the following problem is posed by the method.
FIG. 18 is a simplified schematic view of an integrated laser chip which has been investigated by the inventors prior to the present invention.
A semiconductor chip 1 is monolithically integrated with a plurality of semiconductor lasers 14a through 14d, forward optical guides 15, a multiplexer 16, an amplifier 17 and a modulator 18. Forward beam 60 of the semiconductor lasers 14a through 14d is guided to the multiplexer 16 by the forward optical guides 15 and is irradiated from an emitting face a of the semiconductor chip 1 by being successively guided by the amplifier 17 and the modulator 18. Further, wirings, electrode pads and the like are omitted in the drawing (the same as in following views).
When the plurality of semiconductor lasers 14a through 14d are integrated in the single pitch, in order to avoid interference thereamong, it is necessary to arrange the semiconductor lasers to be remote from each other by a predetermined distance or more. According to the above-described publicly-known example (K.Kudo et al., 25th Optical Fiber Communication Conference, TuL5, (2000)), an area having a width of 80 xcexcm is arranged for arranging 8 pieces of lasers. The range is further increased when a number of integrated lasers is increased.
This signifies that when optical intensities of the respective semiconductor lasers 14a through 14d are monitored, a backward radiation range L becomes large. According to the example of FIG. 18, L becomes about 40 xcexcm. For an optical monitor element constituted by an optical receiving element, it is necessary that an incident position and an incident direction of beam fall within predetermined ranges. The range is about 10 and several xcexcm although depending on the style of the optical monitor. Therefore, when a number of integrating semiconductor lasers becomes large, even in the case of arranging the semiconductor chip land an optical monitor element contiguously to each other, there poses a problem that the range of a position of emitting beam of the respective semiconductor laser exceeds a detectable area (area) of the optical monitor element.
The integrated laser chip (semiconductor chip) 1 is constructed by the constitution of multiplexing, amplifying and modulating forward beam irradiated from the respective semiconductor lasers 14a through 14d and emitting thereof from the extremely small area a at the front end of the chip. Hence, there is conceivable a method of constructing a constitution of dividing a portion of the forward beam and monitoring the forward beam by transmitting the forward beam to an optical monitor without adopting a method of monitoring backward beam. According to the method, although there can be avoided the problem of nonuniformity of monitor accuracy caused by a variation in a position of emitting beam by the respective semiconductor laser, there poses a problem that beam to be used for transmission is attenuated by the monitor. Further, according to the method, there also poses a problem that output of the semiconductor laser cannot be monitored directly since the output of the respective semiconductor laser is monitored through the amplifier.
It is an object of the present invention to provide an optical integrated device, a semiconductor laser module and an optical transmitter capable of highly accurately monitoring output beam of respective semiconductor lasers subjected to laser-array monolithic integration on a semiconductor substrate.
It is other object of the present invention to provide an optical integrated device for emitting backward beam of respective semiconductor lasers subjected to laser-array monolithic integration on a semiconductor substrate from an extremely small area at a backward end of a semiconductor substrate (semiconductor chip).
A simple explanation will be given of an outline of a representative aspect in the present invention disclosed in the application as follows.
Semiconductor optical waveguides (forward optical guides, backward optical guides) for guiding backward beam of respective semiconductor lasers monolithically integrated to a semiconductor substrate to ends of the semiconductor substrate (semiconductor chip ends: forward end and backward end), are integrated to a chip the same as that of the semiconductor lasers, and the respective optical guides are made proximate to each other or bundled by a multiplexer and a range of a position of emitting beam is confined in a detectable area of a used optical monitor to thereby resolve the problem. Different from the semiconductor lasers, relative positions of the optical waveguides can be made proximate to each other or can be multiplexed by the multiplexer. Accordingly, the position of emitting beam can be confined in a predetermined range.
These and other objects and many of the attendant advantages of the invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.